The invention concerns a filling-level measuring device, in particular serving as an overfilling safety means for a liquids container, comprising a non-magnetic slide tube which contains a magnetic switch driven in contactless manner and mounted to a plate or threaded connection sealing a container aperture, further comprising a float displaceable resting on the slide tube and containing a permanent magnet driving the switch, said float being displaceable, by an electromagnetic actuator, independently of the filling level, into a switching position driving said switch into another position.
Filling-level measuring devices of the above kind allow remote-controlled monitoring of their operability. By driving the electromagnetic actuator, the float may be tested for adequate displaceability, and the switch for switching ability.
In a filling-level measuring device of this kind known from U.S. Pat. No. 5,150,614, the electromagnetic actuator consisting of a magnetic coil is mounted on the outside of the plate or threaded connection and the magnetic core is linked to the float by a corrosionproof wire cable passing through a borehole or threaded connection. The cable prevents transmitting the weight of the magnetic core to the float. This known system has been found practical. However it is designed solely for hollow floats and the float's sealing against liquids must be watched by a pressure sensor.
The German Offenlegungsschrift 28 54 284 discloses a magnetically driven float switch for monitoring a liquid level, where the float comprises a permanent magnet for switch actuation and in addition to said magnet a permanent-magnet system which together with a magnet displaceable in a slide tube guiding the float generates magnetic holding when the latter magnet is in the vicinity of the additional permanent-magnet system. A lifting mechanism allows raising the magnet inside the slide tube until the float is liftable on account of said holding from its lower limit position into operational test range. In the rest position, the displaceable lifting magnet is located outside the effective range of interaction with the float-borne magnet-system. This known apparatus is comparatively complex and furthermore it is not intended fro remote control. The permanent-magnet system increases the float weight and entails a larger float volume.
Furthermore the German Offenlegungsschrift 24 42 875 discloses a float/magnetic-switch wherein a solid float per se will not float in liquids of lesser densities than its own. To achieve float buoyancy in the liquid, the float is made to float by the magnetic field of a permanent magnet located at the lower float portion, on the magnetic field of an oppositely polarized permanent magnet affixed underneath the float to the slide tube. If liquid floods over the float, it shall be lifted by the mutually opposing magnetic fields until the magnetic field of the permanent magnet mounted on the float drives the switch inside the slide tube.